Photoionization cross sections for Fe XVIII

School of Mathematics and Physics, The Queen's University of Belfast, Belfast, BT7 1NN, Northern Ireland

Send offprint request to: K.L. Bell

Received:
10
March
1998
Accepted:
3
June
1998

Abstract

A sophisticated
R-matrix calculation is performed for the photoionization of
the fine-structure levels of the ground state of Fe XVIII.
High resolution total
and partial cross sections are obtained with the later restricted to
the cases
of the Fe XVIII ion being
left in one of the fine-structure levels corresponding to
the energetically
lowest 11 LS states of Fe XIX after ionization. Both sets of cross
sections are obtained using
the Breit-Pauli R-matrix approximation which allows for the
possibility of
fine structure splitting in both resonances and thresholds. We find
extensive resonance structure in
the 99 to 113 Ryd photon energy range, a high percentage
of which arises from 2p photoionization leaving the ion in the
2s22p43P2 state while the
background cross section is constructed almost exclusively of 2p and
2s photoionization with the former dominant in both photoionization cases. Relativistic
effects are found to be important in obtaining accurate threshold positions as well
as being responsible for
a significant drop in the background cross section in
comparison with various
calculations done in LS coupling. We believe that this is
the first relativistic calculation for this ion.